Exhaust from combustion engines create nitrogen oxides (NOx) that contribute to smog and other forms of environmental pollution. NOx must be removed from the exhaust streams of these engines in order to protect the environment and satisfy government regulations. Current 3-way catalyst converter technology is used to purify the NOx in automotive exhaust under certain limiting conditions. For example, 3-way catalysts operate at high temperatures greater than 300° C. In addition, in order to meet current emissions standards, 3-way catalysts contain a large quantity of precious metals such as platinum, rhodium, and palladium. Finally, 3-way catalysts only operate properly if the air-to-fuel ratio is stoichiometric.
Several ideas have been proposed to remove NOx from automotive exhaust without using a 3-way catalyst, and to reduce the overall precious metal loading of the catalyst. For stationary combustion sources, such as power plants, waste incinerators, and gas turbines, the most commonly used strategy for NOx purification is selective catalytic reduction (SCR) typically using ammonia (NH3) as a reducing agent over a vanadia/titania catalyst. A similar strategy has been proposed for mobile combustion sources, such as automobiles in which urea is stored on-board and is used as a reducing agent to purify NOx. However, the use of urea on vehicles has limitations, such as the safety of such compounds on a vehicle, as well as a lack of infrastructure for urea distribution.
Other catalysts have also been suggested to replace or enhance the activity of current 3-way catalysts. It has been proposed to use copper-zeolite catalysts (specifically Cu/ZSM-5), however this system was shown to have low activity and durability under exhaust conditions. Additionally, nitrogen storage and reduction (NSR) catalysts may be used to purify NOx. This catalyst combines a 3-way catalyst with an alkaline-based NOx trap. However, this system is very sensitive to sulfur poisoning and the air-fuel ratio must be very tightly controlled.
There is therefore a need in the art for an improved process for the decomposition of NOx in an automotive application.